This invention relates to a method and apparatus for providing a dynamic reference of light in a scanning system, and more particularly to a method and apparatus for measuring intensity of a spot of light in a flying spot scanner for use as a dynamic reference in such applications as film reading.
Using film as a medium for recording scientific data has many advantages. It may be used, for example, to record oscilloscope traces, including A-scope or other radar formats; tracking pictures of missiles or aircraft (theodolite film); astronomical and meterological data; bubble chamber data; medical data; and the like. Film is also an ideal medium for recording traces of very high bandwidth (up to several thousands megacycles) which can not be easily recorded in any other way. In addition, because of the small input power and limited storage space that are required, film is particularly suitable for recording data in space vehicles or aircraft; wind and current measuring devices; and other similar devices.
The problem with film recording has been reading or transcribing the data from film once it has been recorded. It has generally been necessary for an analyst or researcher to read the data visually from the film and transcribe it by hand. This has been found to be a time-consuming, laborious and relatively expensive operation. Consequently, semi-automatic film reading devices have been developed for some applications. However, these semi-automatic film reading devices can read only about five thousand points a day, and require the attention of a human operator.
More recently there have been developed programmable film readers for automatically reading and digitizing very large quantities of film recorded data. A programmable film reader is operated completely under control of a programmed digital computer and therefore does not require the attention of a human operator. Film is read at a rate in excess of five thousand points per second. Data read may be recorded in digital form on magnetic tape for further computer processing and analysis.
The film reading process involves the selected scanning of film by a rapidly moving, programmable light point or spot on a visual display cathode ray tube coupled to the film by suitable optics. The output of this scanning operation is detected by a photosensitive device on the other side of the film. The output of the photosensitive device is coupled to a scan control and monitoring unit for process and analysis comprising a digital computer.
Since the light spot displayed on the cathode ray tube can be rapidly positioned under control of the digital computer, the film reader can be so programmed as to permit the data to be processed and analyzed on a real-time basis with the scanning process. In more conventional so-called "flying spot" scanning techniques, scanning takes place in a predetermined pattern of parallel lines. With line scanning, it is necessary to store the data for all points on each line read until all lines have been scanned before processing of the data can be started. That requires a large segment of computer memory to hole the unprocessed data. Extensive processing is then necessary to extract the significant data.
A true flying spot scanning technique permits a programmable film reader to be controlled by a stored computer program in such a way as to locate and track only the data of interest on the film. No further processing is required; the significant data is immediately available as output of the film reading process. However, locating and tracking only the data of interest requires more sensitive light detection in order to be able to operate over data superimposed on grid backgrounds, "noisy" data, and other complex types of film data. To achieve greater sensitivity, it is necessary to eliminate any false input due to noise and other fluctuations in the intensity of the flying spot.
In optical systems employing a light beam to develop a data signal as the beam passes through a medium, it has sometimes been the practice to split the beam in order to monitor the intensity of the beam with a photometer in the path of one split beam while the intensity of the other split beam directed through the medium is monitored by a main photometer. The problem with this split beam technique is that (i) energy is diverted from the primary path to the reference, (ii) energy is absorbed by the splitting element, and (iii) the rays in the primary path are distorted and require further correction in the imaging lens. Consequently, in applications where all of the available radiation is needed, the split beam technique is not as efficient as a dynamic reference. For example, in a film reader using a cathode ray tube as a flying spot scanner, all of the available radiation from the flying spot is needed to illuminate the film. The problem then is how to develop a dynamic reference in such a system without removing or blocking any light flux in the beam being coupled to the medium.